The present invention relates to a power transmission for automotive vehicles, and more particularly to a power transfer device for four-wheel drive in combination with the power transmission.
As is illustrated in FIG. 3 of the accompanying drawings, a conventional power transfer device of this kind is adapted to a power transmission for front-wheel drive which comprises a differential gear unit 1 of the bevel gear type for front-wheel drive rotatably mounted witin a transmission casing 4, and a hollow shaft 5 rotatably mounted within an extended portion of the transmission casing 4 coaxially with the differential gear unit 1. In such an arrangement, a ring gear 3 for front-wheel drive is fixedly mounted on a gear case 7 of the differential gear unit 1 and permanently in mesh with an output gear 2 of the power transmission, while a second ring gear 6 is fixed to the hollow shaft 5 and permanently in mesh with a drive pinion for rear-wheel drive. The gear case 7 has a hollow shaft portion 8 on which a clutch sleeve 9 is axially slidably mounted by a spline connection to be shifted by a shift fork 10. When shifted toward the hollow shaft 5, as is illustrated in the figure, the clutch sleeve 9 is brought into engagement with a spline 12 of hollow shaft 5 to drivingly connect the gear case 7 with the hollow shaft 5 for four-wheel drive. When shifted toward the differential gear unit 1, the clutch sleeve 9 disengages from the spline 12 of hollow shaft 5 to disconnect the gear case 7 from the hollow shaft 5 for front-wheel drive.
In the power transfer device described above, an original differential gear unit for front-wheel drive may not be adapted as a base for four-wheel drive due to provision of the extended portion of transmission casing 4, the hollow shaft 5, and the hollow shaft portion 8 of gear case 7. For this reason, such a conventional power transfer device is costly in its manufacture. Furthermore, the clutch sleeve 9 is driven by the differential gear unit 1 even in operation for front-wheel drive. This results in power losses of the vehicle engine and defacement of the clutch sleeve 9 due to frictional engagement with the shift fork 10.